The present invention relates to a filler-containing polytetrafluoroethylene granular powder and a preparation process thereof.
Hitherto, a process for preparing a filler-containing polytetrafluoroethylene (PTFE) granular powder by granulating a mixture of a filler and a PTFE powder with stirring in water was proposed, for example, in JP-B-43-8611, JP-B-44-22619, JP-B-48-37576, JP-B-49-17855, JP-B-56-8044, JP-B-57-18730, etc.
However, by the process for preparation disclosed in each of the above-mentioned patent publications, a filler-containing PTFE granular powder having a small particle size and a narrow particle size distribution could not be obtained.
For that reason, in case of preparing, for example, small articles such as seal rings in the form of a rubber band, thin moldings and molded products having a small surface roughness, there was a problem that complicated and uneconomical steps must be employed, such as step of sieving a filler-containing PTFE granular powder to take out only particles of small size and molding them, or step of cutting the obtained molding.
Also a granular powder having excellent powder flowability cannot be obtained only by pulverizing a filler-containing PTFE granular powder.
In JP-B-60-21694, there was proposed a process for preparing a filler-containing PTFE granular powder by granulating, with stirring in water, a PTFE powder and a filler which has been previously surface-treated with an aminosilane compound in coexistence with a water-insoluble organic liquid and an anionic surfactant. However, an apparent density of the filler-containing PTFE granular powder and tensile strength of molded products obtained therefrom are not fully satisfactory.
Further in the above-mentioned preparation process, there were problems that long steps are employed, a complicated step of pre-mixing the filler and the PTFE powder is necessary and cost is high.
The present inventors have made intensive studies in view of the above-mentioned problems, and as a result, have found that those problems can be solved by mixing the PTFE powder and the filler in water in the state of slurry in the presence of a surfactant and further granulating the mixture with stirring in the presence of an organic liquid which forms liquid-liquid interface with water.
The present inventors have made further studies in order to obtain a granular powder more excellent in powder characteristics and physical properties in the above-mentioned granulation process (hereinafter also referred to as xe2x80x9cslurry granulation processxe2x80x9d), and have found that when a specific nonionic surfactant is used, an obtained granular powder has a low electrostatic charge and can give molded articles having a high whiteness (Z value).
Namely an object of the present invention is to provide a filler-containing PTFE granular powder having a good processability and a process for preparation thereof. Particularly an object of the present invention is to provide a filler-containing PTFE granular powder which has a large apparent density, a small average particle size, a narrow particle size distribution, a small electrostatic charge and good physical properties such as powder flowability, and gives molded products having excellent physical properties such as elongation and a high whiteness, and a process for preparation thereof.
The present invention relates to a process for preparing a filler-containing polytetrafluoroethylene granular powder by granulation, in water with stirring, of a polytetrafluoroethylene powder prepared by suspension polymerization and a filler; characterized in that the powder and filler are poured in water separately without being pre-mixed, are mixed with stirring in the presence of a nonionic surfactant having a hydrophobic segment comprising a poly(oxyalkylene) unit having 3 or 4 carbon atoms and a hydrophilic segment comprising a poly(oxyethylene) unit to give a slurry, and then are granulated with stirring in the presence of an organic liquid which forms liquid-liquid interface with water.
The present invention also relates to a process for preparing a filler-containing polytetrafluoroethylene granular powder by granulation, in water with stirring, of a polytetrafluoroethylene powder prepared by suspension polymerization and a filler; characterized in that the powder is poured in water and stirred in the presence of a nonionic surfactant having a hydrophobic segment comprising a poly(oxyalkylene) unit having 3 or 4 carbon atoms and a hydrophilic segment comprising a poly(oxyethylene) unit to give a slurry, and then after the filler is added and mixed to the slurry, the mixture is granulated with stirring in the presence of an organic liquid which forms liquid-liquid interface with water.
Further the present invention relates to a process for preparing a filler-containing polytetrafluoroethylene granular powder by granulation, in water with stirring, of a polytetrafluoroethylene powder prepared by suspension polymerization and a filler; characterized in that a water-containing powder of polytetrafluoroethylene after suspension-polymerized is wet-pulverized without being subjected to drying, poured in water, and stirred in the presence of a nonionic surfactant having a hydrophobic segment comprising a poly(oxyalkylene) unit having 3 or 4 carbon atoms and a hydrophilic segment comprising a poly(oxyethylene) unit to give a slurry, and then after the filler is added and mixed to the slurry, the mixture is granulated with stirring in the presence of an organic liquid which forms liquid-liquid interface with water.
Further the present invention relates to a process for preparing a filler-containing polytetrafluoroethylene granular powder; characterized in that in each of the above-mentioned preparation processes, the granulation with stirring is carried out in combination use of stirring and deagglomeration by using means for deagglomerating a granular powder obtained by the granulation in addition to means for stirring.
The present invention also relates to a filler-containing PTFE granular powder prepared by any of the above-mentioned processes which is characterized in that an apparent density of the filler-containing PTFE granular powder is not less than 0.7 g/cm3, a flowability (defined hereinafter) of the granular powder is not less than 6 times and an electrostatic charge is not more than 50 V or that an angle of repose is not more than 40 degrees in case of an apparent density of not less than 0.7 g/cm3 and less than 0.9 g/cm3, not more than 38 degrees in case of an apparent density of not less than 0.9 g/cm3 and less than 1.0 g/cm3, and not more than 36 degrees in case of an apparent density of not less than 1.0 g/cm3, and an average particle size is not more than 500 xcexcm and an electrostatic charge is not more than 50 V.